Acicular chromium dioxide, its preparation and the use of this material for magnetic recording media have been described in many publications. Compared with recording media based on other magnetic oxides, magnetic recording media which contain chromium dioxide generally have superior magnetic properties which are due to the high values of the coercive force and of the specific remanence and saturation magnetization and in particular to the uniform shape and the small dimensions of the acicular chromium dioxide particles.
The development in the processing of analog and in particular digital audio and video signals, which has now made further progress, and increasing requirements with regard to the thermal stability of the magnetic recording also necessitate correspondingly improved magnetic recording media for the storage thereof. In the case of magnetic recording media which contain magnetizable particles, for example chromium dioxide, in the binder matrix of the magnetic layer, this means that, on the one hand, an increase in the coercive force of the magnetic particles and a reduction in their particle size are necessary in order to increase the storage density. In addition, it should be ensured that the magnetic moment of the particles remains constant or is increased if possible so that the residual flux of the magnetic layer, which is decisive for the recording level, is maintained. On the other hand, independently of the level of the coercive force, it should be ensured that the particle size distribution of the pigment sample remains very narrow since the thermal stability of the residual flux is also determined by this. Moreover, it should be guaranteed that the coercive force distribution, which is determined not only by the particle size but decisively by the distribution of the dopants used, remains narrow. In the case of a broad distribution, the low-coercivity fraction, which in some cases is identical to the smaller particles of the size distribution, tends to undergo thermal demagnetization more readily than the high-coercivity fraction. Finally, the degree of thermal demagnetization is influenced by the level of the Curie temperature of the pigment.
There has therefore been no lack of attempts to push ahead the development of chromium dioxide in the direction of higher coercive force but, independently of the coercive force, also toward higher saturation magnetization and at the same time a narrower switching field distribution and particle size distribution.
Processes for the preparation of CrO.sub.2 pigments having coercive forces of more than 61 kA/m are described in DE 26 48 305, DE 38 37 646 and EP-A 0 548 642, the chromium dioxide prepared according to EP-A 0 548 642 most closely approaching the desired property profile and representing the preamble of the present invention.
The stated EP-A 0 548 642 describes a process for the preparation of CrO.sub.2 modified with iron and tellurium and/or antimony and having a coercive force of more than 60 kA/m, a saturation magnetization of at least 85 nTm.sup.3 /g and a very narrow particle size distribution whose mean deviation from the average needle length is less than 35%. Although these chromium dioxide materials prepared according to EP-A 0 548 642 are suitable for the future high-density recording media and storage methods, there is still a need for products further improved with regard to the coercive force distribution and hence the heat stability, as revealed by corresponding temperature-dependent measurements of the residual induction and of the susceptibility.
It has been found that the half-width .DELTA.T of the signal peak observed in a measurement of the susceptibility is a measure of the temperature range in which the gradual thermal demagnetization of the sample takes place. The determination of the half-width is shown in FIGS. 1 and 2. A narrow peak therefore represents a uniform sample which contains only small amounts of particle fractions switching at low temperatures, whereas a broad peak shows that a broad particle size distribution is present, with a correspondingly broadly distributed range of thermally switching particles. A measure of the distribution width of particle fractions switching at high temperatures is the slope of the tangent T to the curve 2 (cf. FIGS. 1 and 2). A gentle slope represents a broad distribution.
In the temperature-dependent measurement of the residual induction M.sub.r, the more non-uniform the sample, the more sharply the M.sub.r decreases in the initial phase, as is evident from a comparison of the Figures. A temperature (T.sub.50%) at which a certain M.sub.r, for example 50% of the original M.sub.r, as shown in the examples, is still present is therefore a measure of the uniformity. However, it should be noted that the particle length is included in this consideration. It is known that larger particles lose residual induction at higher temperatures (thermal activation).
The T.sub.50% values and half-widths .DELTA.T, mentioned in the subsequent Examples, of samples according to the pigment type described in EP-A 0 548 642 indicate very broad distributions. This pigment type may be assumed to be typical of all CrO.sub.2 pigments obtainable according to the prior art and having coercive forces greater than 60 kA/m.
Low-coercivity, iron-doped CrO.sub.2 pigments which can be prepared, for example according to European Patents 0,198,110, 0,146,127, 0,239,089 and 0,304,851, by hydrothermal conversion of CrO.sub.3 and Cr.sub.2 O.sub.3 or of chromium(III) chromate having coercive forces of less than 46 kA/m show on the other hand a narrower distribution since only very small amounts of iron, for example less than 0.5% by weight, based on the theoretical CrO.sub.2 yield, of Fe.sub.2 O.sub.3, have to be used for such pigments and hence a very inhomogeneous distribution of the iron dopant is not to be expected, and, owing to their coarse particles, these pigments have high thermal activation, for example as indicated by the T.sub.50% value and the half-width of Comparative Example 4 carried out according to European Patent 0,198,110. On the other hand, the distribution of the coercive force is broader if more than 0.5% by weight, based on the theoretical CrO.sub.2 yield, of Fe.sub.2 O.sub.3 has to be used to achieve coercive forces of up to 60 kA/m, as indicated by the T.sub.50% values and half-widths documented in Comparative Examples 5 and 6 according to European Patent 0,198,110. European Patent 0,146,127 describes a process in which the coercive force distribution can be improved by using Fe.sub.2 O.sub.3 dissolved in aqueous CrO.sub.3, but no measured values are given to support this quantitatively.
The common feature of all processes described for the preparation of CrO.sub.2 having coercive forces of greater than or less than 60 kA/m is that iron oxides or inorganic iron salts, for example iron sulfate, are used for the doping with iron.
It is an object of the present invention to provide a process for the preparation of modified chromium dioxide having a distribution of the particle size and of the coercive force which is narrower compared with the prior art and consequently having improved heat stability of the magnetization in combination with coercive forces of at least 40 kA/m.